In mobile communication networks, there is always a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the mobile communication network is deployed.
In national security and public safety (NSPS) scenarios, there is a need to allow wireless devices (such as cellular user equipment, UE, modems, smartphones, sensors, tablet computer, machine type devices) to communicate directly with one another when they are under cellular network coverage. In 3GPP LTE networks (NW), this so called Device to Device communication (D2D) or Proximity Services (ProSe) is made possible by the “LTE Direct” technology that allows a cellular base station (BS or eNB) to set up a direct D2D link between two UEs and allocate resources for that link.
D2D communication exists in several versions today. Examples include Bluetooth, several variants of the IEEE 802.11 standards suite such as WiFi Direct, and Flashlinq. Recently, device-to-device communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. An advantage with D2D under cellular a cellular NW control (network assisted D2D) is that optimized spectrum usage as well as low power discovery algorithms can be developed.
A technical mechanism that addresses D2D communication both in and outside NW coverage is based on clustering, where some of the devices (UEs) act as a Cluster Head (CH) device and other devices act as Slave devices. A CH device can be compared to a small range base station that, in the absence of a cellular eNB, provides (a subset of and) similar functionality as an eNB. For example, a CH device may provide synchronization and radio resource management within the cluster and may also act as a central node to create a tree topology for communication with the cluster. In addition, the CH device may also provide a relaying functionality towards other clusters or towards a cellular eNB.
In a cluster scenario, wireless devices within the cluster are required to do cell search on regular basis in order to be able to detect whether the wireless device has come into NW coverage again. Once a wireless device (or the CH device itself) determines presence of a regular network node, the cluster has an operative connection to the core network (and Internet Protocol network) and more services may be able to be supported within the cluster.
In general terms, CH devices are low power nodes, often battery driven, and assumed to have lower cost compared to eNBs (or other regular NW nodes). Therefore, the CH devices will be made up by simpler components with lower performance compared to that of an eNB. One such component is the crystal oscillator (XO), which is used for generating reference timing and reference carrier frequency. Current NW nodes have a very accurate XO and are assumed always to be connected to a power supply (such as a regular power supply, solar power, a powerful battery backup, etc.). The requirement for the frequency accuracy for a NW node is 0.1 ppm, corresponding to a frequency error of less than 200 Hz at a carrier frequency of 2 GHz.
In current wireless devices (mobile phones, tablets, etc.) the XO accuracy is 10-15 ppm implying a frequency error of ±20-30 kHz at 2 GHz. Hence, once a wireless device at start up connects to a NW node, the wireless today needs need to not only to find a NW node (a cell) to camp on but also to find the correct carrier frequency and lock its interval frequency generation to a more accurate source than the internal XO of the wireless device. Known Cell Search (CS) algorithms aim for determination of specific synchronization signals transmitted from the NW nodes in order to determine timing, the exact carrier frequency, as well as the NW node cell ID. Synchronization signals (PSS/SSS in LTE, P-SCH/S-SCH in WCDMA) used for determining cells in current cellular systems like LTE and WCDMA/HSPA work with a frequency error of up to approximately 3-4 kHz. With this robustness of CS, a wireless device can perform CS using its own carrier frequency in connected mode (i.e. when in-sync with the NW), also for the highest reasonable Doppler shift. However, during initial CS, i.e. when a wireless device has not yet connected to any NW node, a frequency grid for the CS is needed with a 5-10 kHz carrier spacing in order to cope with the uncertainty of the XO.
CH devices may have XOs with accuracies closer to today's UEs than today's NW nodes, mainly due to cost and power consumption reasons. This means that the devices in the network need to operate with regular network nodes (such as eNBs) and low power network nodes (such as CH devices) that have different XO accuracy.
Hence, there is still a need for an improved carrier frequency handling in heterogeneous communications networks, such as cluster based communications networks.